1. Field of the Invention
The present invention relates to a hollow die and an apparatus for continuous extrusion forming of hollow articles made of a metal such as aluminum.
2. Prior Art
The so called, continuous extrusion forming apparatus is known in the art. This apparatus may be used to continuously extrude hollow metal articles, such as aluminum tubes `E`. Those tubes are, for example, of a profile as shown in FIG. 8, and are used in manufacture of heat exchangers.
FIG. 10 shows the principle of the continuous extrusion forming apparatus. This apparatus comprises an extrusion wheel 52 having an annular groove 51 formed therearound. A wire or the like material `M` for extrusion will be guided along this groove of the continuously rotating wheel 52. Shoes 53 are in a sliding contact with the outer periphery of the wheel, so as to accommodate feed plates 54 and 55 in place. The feed plates are arranged fore and aft to extend a distance around the wheel, and cover the annular groove 51 to form a pressure chamber within the shoes. A die 57 is held outside the innermost feed plate 54, and an abutment 59 is positioned ahead this plate circumferentially of the wheel 52. A foot of the abutment 59 protrudes into the annular groove 51. A short enlarged groove 60 formed in the innermost feed plate 54 faces and cooperates with the annular groove, thereby providing the pressure chamber with a sufficient space.
The continuous extrusion forming apparatus outlined above will operate as follows. The wheel 52 whose groove 51 is guiding the wire-shaped material M is driven to rotate and force this material in between the wheel and the feed plates 54 and 55. Consequently, the material will be compressed in the pressure chamber 61 defined between the wheel 51, the feed plates 54 and 55 and the abutment 59. The material thus compressed will be extruded through the die 57 to produce an extrudate of a desired configuration.
In general, the die assemblies used in the continuous extrusion forming apparatus have been split dies 57 each composed of a male die 62 and a female die 63. As shown in FIGS. 9A and 9B, the male die 62 comprises a short and thick columnar body 64 and a bridge 65 integral therewith. This bridge 65 crosses a cavity defined through and centrally of the body. A core 66 having a bearing tip 67 for defining a hollow longitudinal space through the extrudate is secured in the bridge, such that the tip protrudes forwardly of the male die. On the other hand, the female die 63 Comprises a thick disc 69 of the same outer diameter as the columnar body 64 of the male die. A forming hole 70 is formed through the thick disc so as to define the outer periphery of said extrudate. In this split die, the male die 62 is located behind the female die 63 in the direction of extrusion.
A higher extrusion speed which has been desired to raise productivity, inevitably cause a stronger stress to be imposed on the die 57. Such a stress will bend the die and result in an irregular configuration of the forming slit 71, thus failing to produce high precision extrudates.
This problem is not necessarily inherent only in the continuous extrusion forming apparatuses each having the die 57 as described above. However, as shown in FIG. 11, the innermost feed plate 54 in said apparatus tends to get warped due to the high pressure of material `M` compressed in the enlarged groove 60. FIGS. 9A and 9B illustrate that such a deformation of the plate 54 will bring about an asymmetrical deformation of the die 57. A distorted forming slit 71 in the die causes a serious defect in the dimensional preciseness of extrudates `E`.